Topic: Why do Iodine crystals sublime? Read times. Why do Iodine crystals sublime? And here is the answer: They sublime because though there are covalent bonds within the molecule holding two iodine atoms together, only van der Waal's forces exist between the iodine molecules. Van der Waal's forces are very weak interactions between molecules of a substance hence the iodine molecules can escape from the solid structure easily. Is it right?
What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? A phase diagram of a single substance describes the behaviour of that substance as the pressure and temperature are changed.
Imagine a sample of solid iodine in an idealized syringe that can withstand any temperature and pressure, and is transparent, so you can see what is happening inside. No other substance is present — no air, water, etc. The system must be at equilibrium, with a uniform temperature and pressure throughout. Suppose that you start with the syringe at All of the iodine is in the solid state.
Now slowly warm the sample, adding energy at a constant rate, while the pressure stays constant. When the temperature gets to When melting is complete the temperature resumes its rise until it gets to When vaporization is complete, the temperature again begins rising. In this system, we see iodine vapour only when the temperature is at In contrast, when we observe a sample of solid iodine in the bottom of an unsealed glass flask, at 20 o C and 1 atm pressure, the system is different from the idealized system just described.
There is air in the flask, and we see a faint colour of iodine vapour. Although the vapour pressure of iodine is very low 0. Thus, the notion that upon heating iodine only sublimes, but does not melt is present in many chemistry textbooks, teachers lectures and, therefore, in students minds and may be considered as one of the widespread misconceptions in chemistry teaching. In this paper we offer a lecture demonstration showing the existence of all three states of iodine, supported by a short video-clip, hoping to give a contribution to the correction of misbelieves about the process of sublimation and the examples of subliming substances.
Keywords: sublimation, misconceptions, textbooks, experiments, iodine, chemistry teaching. There are several terms that refer to students' misbelieves. Some authors use the word "misconception" to define erroneous notions and others use "preconceptions" that are related to previous knowledge or arise during the course of instructions. The expression "alternative conceptions" is considered by some authors as some kind of compromise or agreement that incorporates students' faulty views during science teaching Horton, , p.
The process of previous learning plays an important role in students' understanding and the quality of the subsequently learned concepts Roschelle, A large number of students and some teachers, too believe that their established concepts are correct because they make sense , meaning that they correspond to their understanding of the phenomenon in question.
Consequently, when students face new information which, unlike their alternative conceptions, does not fit their previously established mental framework, they may ignore it or reject it because it seems wrong Horton, , p. They attempt to solve problems in chemistry courses without real understanding of a process or a phenomenon connecting them with their previous information and concepts, which, however, may not be scientifically correct.
Students can be very successful and intelligent; they may have high grades, but still retain certain misconceptions. Identifying the weaknesses in the concept-building is especially important during the students' first exposure to chemistry. The misconceptions they build in the early stages of their development are the most resistant to change during the subsequent instruction, the students constructing the new knowledge on a faulty basis and rearranging the new information and ideas to fit the framework of ideas they believe are correct.
Thus, it is of utmost importance to identify, confront and correct different misconceptions that students have.
The knowledge of students' misconceptions is helpful in deciding where to start and how to continue teaching. It is interesting but also disturbing that some of the basic concepts and terms used in the chemistry education from the earliest stages up to the university level are not properly, precisely and unequivocally defined and seem to have different meanings for different people. Rather surprisingly, the concepts of sublimation and subliming substance seem to fall into this category.
It would be applicable to any solid, at any pressure or any temperature above 0 K, the possible differences being only quantitative and dependent on the vapor pressure of the solid in question.
Indeed, such a broad and loose definition of sublimation is widely found in textbooks and other sources of chemical information. When examples of subliming substances are considered, the most usually quoted ones are dry ice solid carbon dioxide , iodine and naphthalene.
Carbon dioxide is an example of a substance that sublimes and On the other hand, many articles can be found Wisconsin State Journal , ; Habby, ; Wikipedia , ; Silberberg, , about the process of sublimation of snow and ice which sublime, albeit slowly, below the melting-point temperature.
This phenomenon is operative for example when linen are hung wet outdoors in freezing weather to be retrieved dry at a later time. The loss of snow from a snowfield during a cold spell is often caused by sunshine acting directly on the outer layers of the snow.
Ablation is a process which includes sublimation and erosive wear of glacier ice. The snow sublimes through a process that is similar to evaporation. In fact, whenever there is an interface of air and water either liquid or solid , the H 2 O molecules will have some tendency, more or less pronounced, to leave the condensed phase and the processes of water evaporation and sublimation are observable at any temperature.
Clearly, this is nothing new or spectacular but we do not think of water as a typical example of a subliming substance since ordinarily ice first melts and then vaporizes. Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers, and students in the field of chemistry.
It only takes a minute to sign up. Connect and share knowledge within a single location that is structured and easy to search. Why does iodine sublime? I have researched it myself and I have got the same answer; it sublimes because it directly converts from solid to gas.
But why don't bromine or chlorine also sublime? Iodine sublimes for the same reasons that all solids do: because it has some equilibrium vapor pressure an normal conditions. Now, the value of that pressure varies greatly in different solids.
For many of them, it is so extremely low that for all practical reasons it can be considered non-existent. Say, if you'd leave a sample of NaCl for a thousand years, I don't think you will be able to detect the weight loss due to sublimation.
For iodine, the vapor pressure is significant, so a sample left in the open will vanish completely in a matter of hours. Some sources may specify that iodine goes directly from solid to gas when heated, never becoming a liquid.
That's an urban legend; iodine does have the melting point and the boiling point at standard pressure, and between the two it will be liquid. It is not easy to demonstrate, though; if you heat it in the open, it will probably sublime away even before reaching the melting point, and if you do it in a closed container, the vapors get so thick you can't see a thing.
That being said, there are indeed some compounds that would never turn liquid at ambient pressure.
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